Electrolytic recovery of metals from solution is well known. Such electrolytic recovery is disclosed in U.S. Pat. Nos. 3,785,950; 3,535,218; 1,839,905; and 3,579,431.
The present invention is particularly suited for the removal of metals from solution of low metal content where the metal will deposit on a cathode in a powdery form. An example of this is the removal of copper from etching solutions used on printed circuit boards and the invention will be described in that environment. In the etching of printed circuit boards using one common etchant, copper is introduced in the etchant and cupric or cuprous ammonium chloride is produced. Copper can be efficiently recovered from this used etchant solution and also etchants which contain concentrations of cupric or cuprous ammonium salts. By removing the copper electrolytically, the etching solution can be substantially regenerated. However, the processed etchant still contains some copper in solution and the solution may be objectionable as an etchant. In one copper recovery technique, a cell containing a plurality of alternate anodes and cathOdes is utilized where the electrolyte is ammoniacal ammonium chloride. The anodes are made of graphite and the cathodes of copper. Oxygen is liberated at the anode and the following effective reaction occurs at the cathode. EQU Cu(NH.sub.3).sub.4 Cl.sub.2 +H.sub.2 =Cu+2NH.sub.4 Cl+2NH.sub.3
The outflowing solution, therefore, becomes a regenerated etchant, but, however, still contains some copper in solution.
In the described recovery process, copper is deposited in powdery form on the cathode due to the electrolysis occurring within the cell and the outflowing solution becomes a regenerated printed circuit board etchant of substantially reduced copper content. The copper powder builds up on the cathodes, and occasionally, the adhered powder falls to the bottom of the cell. The copper powder collected in the bottom of the cell is occasionally removed by pumping, or any other conventional method. A system of the type described will provide an electrolyte output containing approximately two grams/litre copper while the etchant introduced into the cell to be treated contains about one hundred and twenty grams per litre.
The present invention provides a system for secondary treatment of the etchant which will remove additional copper in powdered form and provide a clarified etchant having less than one part per million copper. The finally treated etchant, after NH.sub.3 addition, is essentially equivalent to a virgin etchant.
The metal content of a solution may be reduced by primary treatment using other techniques such as ion exchange, hydrogen reduction and solvent extraction, as well as electrolysis.